inverse square law (Score 1)

drag goes up as the square of speed.

power required goes up as the cube.

do the dimensional analysis and you'll find that fuel economy goes down as the inverse square of speed. 120mph is 4 times worse than 60mph.

in the limit this is absolutely true, but at lower speeds there are other factors. mileage in my civic hybrid peaks at about 45 mph (~60 mpg), and i still get 50mpg at 65mph.

YMMV